1. Field of the Invention
The present invention relates to an optical element transmitting light, a lens unit and an image pickup apparatus having subject light focused to obtain image data.
2. Description of the Related Art
There are cameras having a zoom function of freely setting a shooting field angle among electronic still cameras providing an image of a subject on a solid-state image sensing device such as a charge coupled device (CCD) and retrieving image data representing the subject as a signal and film cameras doing a photo shoot on a photographic film. Such cameras are equipped with an image taking lens of which focal length varies according to operation of a zoom switch. The image taking lens is generally a compound lens composed of a combination of multiple lens elements, where relative positions of the multiple lens elements are adjusted according to the focal length set up by the zoom switch. Such cameras have a cam mechanism. The cam mechanism conveys rotations of a motor according to the operation of the zoom switch, and the multiple lens elements thereby move back and forth in an optical axis direction respectively so as to adjust the relative positions and change the focal length.
There is a focus lens for focus adjustment among the multiple lens elements. There are the cases where a lens drive mechanism moving this focus lens is provided separately from the cam mechanism.
In recent years, there has been a proposal of a liquid lens of a variable focal length having two kinds of liquid of mutually different refractive indexes and mutually unmixable accommodated therein instead of the image taking lens having the drive mechanism (for instance refer to Non-patent Document, “Philips' Fluid Lenses”, [online], Mar. 3, 2004, Royal Philips Electronics, [searched on Mar. 31 ,2004],Internet, hereafter referred to as Non-patent document 1.)
The liquid lens proposed in Nonpatent Document 1 has two kinds of liquid of mutually different refractive indexes and mutually unmixable accommodated therein. One of the two kinds of liquid is a conductive solution having a supporting electrolyte dissolved therein, and the other liquid is insulating oil. These liquids are accommodated in a liquid container which is composed of a short glass tube having both ends thereof closed up by light-transmitting transparent end caps. Inner walls of this tube and one of the end caps are covered with a water-repellent film. In the case of the liquid lens thus configured, of the two kinds of liquid, the conductive solution repels the inner walls of the tube and one of the end caps covered with the water-repellent film. And the conductive solution stays in a semispherical form in contact with the other end cap, and so an interface portion between the conductive solution and the insulating oil functions as a concave lens. The liquid lens also has two electrodes for the sake of applying a voltage to the conductive solution, where one of the two electrodes is placed to contact the conductive solution while the other electrode is placed in the back of the water-repellent film. If the voltage is applied to such electrodes, electric charge is emitted from the electrode placed to contact the conductive solution into the conductive solution so that there occurs a phenomenon in which the emitted charge stays in the interface portion with the insulating oil in the conductive solution. The charge staying in the interface portion and the charge of reverse polarity to that charge gathered on the electrode placed in the back of the water-repellent film draw each other with a coulomb force so as to attract the charge in the conductive solution around the water-repellent film. Consequently, the conductive solution starts to wet the water-repellent film covering the inner wall of the tube, causing change of an interface form of the two kinds of liquid. To be more specific, as the higher voltage is applied to the conductive solution, a change occurs to a curvature radius of the interface portion with the insulating oil of the conductive solution initially functioning as the concave lens. For instance, the interface portion becomes completely flat or the conductive solution starts to function as the convex lens, resulting in a change in the focal length.
According to such a liquid lens, it is possible to change the focal length without moving the lens, so the zoom function and a focus function can be performed without providing the cam mechanism and lens drive mechanism. Therefore, by using the liquid lens, the apparatus can be substantially downsized, and the lens can also be applicable to a small device such as a cellular phone.
According to the liquid lens described in Nonpatent Document 1, however, the conductive solution is electrolyzed by the electric charge emitted from the electrode. Therefore, there is a problem that, if used for a long period of time, a generated gas stays in the liquid container and turns into bubbles, scattering of light occurs and light transmission lowers.
There is also a problem that, if the liquid lens described in Nonpatent Document 1 is used in a car in midsummer, the conductive solution becomes volatile in a temperature environment of approximately 100 degrees and performance of the lens is thereby deteriorated.